The reforming catalysts for hydrocarbons usually contain nickel as the active element; the nickel being added either (1) by dipping the pre-shaped ceramic structures in nickel-salt solutions; these ceramic structures may contain aluminium, magnesium, silica, alcaline oxides, titanium, CaO, and rare earth oxides may be used as stabilisers. In addition, for particular applications of heavier hydrocarbon feeds, the nickel may be promoted with the oxides of iron and manganese, and optionally barium or (2) by mixing or coprecipitating the nickel or its compounds prior to shaping the catalyst.
Usually reforming catalysts are presented in the form of solid cylinders, cylindrical rings, spheres or granules.
In the last few years particular attention has been given to the size and shape of the catalysts. The form and size of the catalysts have an important effect on pressure drop and catalyst activity, and on its possibilities as concerns heat transfer and crush strength.
A new catalyst form is described in French Pat. No. 2,328,656. This patent describes a catalyst composed of a hollow cylinder with axial divisions; this arrangement effectively presents a much greater contact surface, which automatically increases catalyst activity since one of the limiting factors of the velocity of the catalyst reaction is the diffusion of the reactants of the catalyst's elements; furthermore, this structure makes it possible to obtain lower pressure drop in the reforming reactor, whether it be of the tubular or adiabatic types.
Four main characteristics are expected from a reforming catalyst:
1. Good mechanical resistance: this characteristic is particularly important in the tubular reforming furnaces where the catalyst is subjected to severe contraints, particularly at shutdowns of the furnace. Thus, for a catalyst having a given chemical composition and microscopic structure, the size and shape of the catalyst have a determining effect on the mechanical resistance of the catalyst.
This mechanical resistance is measured by the crush strength and resistance to shocks.
2. High Activity: This characteristics makes it possible to reduce the required volume of catalyst, or with equal volumes to improve the performance of the catalytic reactor. Further, in the case of tubular reformers, high activity makes it possible to reduce tube temperatures and thus the energy consumed in heating.
Since the diffusion velocity of the gases in reaction is a limiting factor in the velocity of catalysis, it is preferable, for a catalyst with a given chemical composition and microscopic structure, to increase the contact surface between the reacting gas and the catalyst.
A classic way to obtain a large surface contact is to use small catalyst elements; for example cylindrical rings with a small diameter and height--but this will provoke an increase of the pressure drop--which can become prohibitive.
3. A high heat transfer coefficient: this characteristic is important in the tubular reforming furnaces. A good transfer coefficient allows a reduction in the volume of catalyst required or a reduction of the operating temperatures and energy consumption. This phenomenon of heat transfer taking place through the tube wall and through the piling up of the catalyst in the tube from the outside tube surface is a complex phenomenon which introduces different types of heat transfer.
A small pressure drop, but nonetheless enough to ensure a suitable distribution of the reacting gases in the tubes of the tubular furnaces or in the catalytic mass of the adiabatic reactors is advantageous for a catalyst with a given chemical composition and microscopic structure, this can be obtained by increasing the particle sizes of the catalyst, by increasing the void space in the catalytic bed and by decreasing the friction surfaces between the gas and the catalyst's particles.
As can be seen from the above, for a catalyst with a given chemical composition and microscopic structure, the shape and size have a considerable effect on the catalyst behaviour. Changes in these two characteristics can have contradictory influences on its behaviour, e.g. if we consider a spherical shaped catalyst, we will notice that a decrease of the diameter of the spheres will have an unfavourable influence on the pressure drop and the overall rate of heat transfer, but a favourable influence on the activity of the catalyst.
Moreover, for a catalyst of a specified chemical composition and microscopic structure, the optimum choice of size and shape of the catalyst particles will depend not only on the operating conditions, but also, for example, in the case of tubular reactors, on the diameter of the tubes of the reactor.
This invention concerns a new catalyst form which can be produced cheaply, which presents a high contact surface, which gives a relatively low pressure drop, which ensures a high heat transfer and which offers a good mechanical resistance.